The present invention relates to a co-catalyst for purifying an exhaust gas of an internal combustion engine, more particularly to the co-catalyst for efficiently converting the exhaust gas containing an environmental polluting substance such as carbon monoxide, hydrocarbons and nitrogen oxides into a nontoxic substance by means of oxidation and/or reduction.
A catalyst prepared by finely dispersing a precious metal such as platinum, rhodium and palladium on a support such as alumina having a larger specific surface area is employed for purifying an exhaust gas discharged from an internal combustion engine of an automobile. These precious metals have a role of converting the nitrogen oxides into nitrogen in addition to a role of converting the hydrocarbons and the carbon monoxide in the exhaust gas into carbon dioxide and water and into carbon dioxide, respectively, by oxidation. A simultaneous catalysis on both of the oxidation reaction and the reduction reaction enables simultaneous removal of the three components including the hydrocarbons, the carbon monoxide and the nitrogen oxides. It is required to maintain a ratio between fuel and air (air-fuel ratio) at constant (at a theoretical air-fuel ratio) for efficiently affecting the simultaneous catalysis on the both reactions.
However, the number of rotations of the internal combustion engine of the automobile is likely to be changed instantaneously, and the air-fuel ratio largely changes depending on driving situation such as speed-acceleration, speed-reduction, lower-speed driving and higher-speed driving. Accordingly, an amount of fuel supplied to an engine is controlled to make the air-fuel ratio constant by judging the change of an oxygen concentration in the exhaust gas by using an oxygen sensor.
The precious metal catalyst is employed with a co-catalyst for a purpose of preventing the reduction of the purifying catalysis generated due to the change of the air-fuel ratio by the chemical function of the catalyst itself. An example of the co-catalyst includes cerium oxide. The cerium oxide has a characteristic of eliminating and absorbing adhered oxygen and lattice oxygen in the cerium oxide depending on the degree of the oxygen partial pressure. Accordingly, when the exhaust gas is reductive, the cerium oxide eliminates the oxygen [CeO2xe2x86x92CeO2-x+(x/2)O2] to supply the oxygen into the exhaust gas for affecting the oxidation reaction. On the other hand, when the exhaust gas is oxidative, the cerium oxide taking the oxygen into its oxygen defects [CeO2-x+(x/2)O2xe2x86x92CeO2] to decrease the oxygen concentration in the exhaust gas for affecting the reduction reaction. In this manner, the cerium oxide acts as a buffering agent for decreasing the change of the oxidative property and the reductive property of the exhaust gas to maintain the purifying property of the catalyst.
However, the co-catalyst for purifying the exhaust gas is likely to be deteriorated because of the long-time exposure to a higher-temperature gas from the internal combustion engine. Especially, the resistance to heat of the cerium oxide is low, and the cerium oxide is sintered to reduce its specific surface area upon exposure to a higher-temperature gas, thereby resulting in the reduction of the initial properties as the co-catalyst. Accordingly, the elevation of the thermal stability by the addition of other elements such as zirconium or of the stability of the hexagonal by increasing the lattice constant by means of the addition of other elements is attempted. However, the satisfactory results have not jet been obtained.
Although the cerium oxide is mixed with aluminum oxide (xcex1-type, xcex3-type, xcex8-type) to prepare a co-catalyst in accordance with a widely-employed technology and the sintering of the mixture can be physically prevented by the mixing of the aluminum oxide in this case, the sintering of the cerium oxide itself cannot be prevented.
Even if the sintering of the cerium oxide could be prevented, the actively is lowered in a relatively short period of time not to obtain a substantially longer lifetime when the co-catalyst is continuously used with the precious metal catalyst in an actual internal combustion engine for purifying an exhaust gas.
An object of the present invention is to provide a co-catalyst for purifying an exhaust gas which can be used for a loner period of time as an actual catalyst and a process for treating an exhaust gas catalyst by using the cerium oxide in the conventional co-catalyst for purifying the exhaust gas as a cerium-containing complex oxide for elevating the resistance to heat and suppressing the performance reduction due to thermal deterioration and by making a specific surface area and an oxygen storage capacity over specified values.
The present invention is a co-catalyst for purifying an exhaust gas including; a composite oxide including (a) cerium; and (b) at least one element selected from the group consisting of zirconium, yttrium, strontium, barium and a rare-earth element supported on a particulate aluminum oxide support; a specific surface area of the co-catalyst after sintering being not less than 40 m2/g; an oxygen storage capacity at 700xc2x0 C. being not less than 100 xcexcmols/g or an oxygen storage capacity at 400xc2x0 C. being not less than 10 xcexcmols/g.
The present invention will be described in detail.
As described above, the sintering of the cerium oxide itself cannot be prevented even if the co-catalyst is formed by mixing the cerium oxide and the aluminum oxide. However, the repeated experiments by the present inventors revealed that a co-catalyst containing a cerium-based composite oxide having a higher stability to heat and an anti-sintering property prepared by supporting a composite oxide containing cerium, a specified element and oxygen on the particulate aluminum oxide. The co-catalyst can be prepared, for example, by reacting an aqueous solution dissolving therein a water-soluble salt of cerium and another water-soluble salt of a specified element in contact with the particulate aluminum oxide, with a specified precipitating agent (reducing agent) to deposit the reaction product on the particulate aluminum oxide and sintering the particulate aluminum oxide having the deposited reaction product thereon to support the composite oxide containing the cerium, the specified element and the oxygen on the particulate aluminum oxide.
It has been known that the use of the co-catalyst having the specific surface area and the specified oxygen storage capacity among those having the above particulate aluminum. oxide supporting the composite oxide can elevate the catalysis and increase the treating efficiency of the actual exhaust gas.
After vigorous experiments for elevating the catalysis, the present inventors have conceived based on the above knowledge that the co-catalyst can be prevented from the reduction of the initial activity even after exposure to a higher temperature for a longer period of time to maintain the higher catalysis by supporting the cerium-based composite oxide containing the cerium oxide and an oxide of another metal on the particulate aluminum oxide and by adjusting the specific surface area thereof after higher-temperature sintering to be 40 m2/g or more, and the oxygen storage capacities at 700xc2x0 C. and 400xc2x0 C. to be 100 xcexcmols/g and 10 xcexcmols/g, respectively, thereby reaching to the present invention.
A composition ratio between the aluminum oxide and the composite oxide, and a composition ratio between the cerium and the added element in the composite oxide are not especially restricted. However, the reduction of a relative amount of the cerium in the co-catalyst tends to decrease the effect of the co-catalyst. When, inversely, the relative amount of the cerium is excess, the cerium oxide separately exists in addition to the composite oxide containing the cerium or all the cerium oxide cannot be supported on the aluminum oxide. Accordingly, the excessive relative amount of the cerium does not produce the effect corresponding thereto. The composition ratio between the aluminum oxide and the composite oxide is preferably about (1:0.5) to (1:4) by weight. The composition ratio between the cerium and the added element in the composite oxide when calculated into the corresponding oxides is preferably about (1:0.1) to (0.1:1), and more preferably about (1:0.2) to (0.2:1).
The aluminum oxide employed in the present invention is desirably particulate, and has the larger specific surface area and the higher stability to heat.
Examples of such aluminum oxide include xcex1-alumina, xcex8-alumina and xcex3-alumina, and a foreign element (for example, an alkaline earth metal and silica) may be added to the alumina. However, boehmite which is included in the alumina is excluded form the alumina of the present invention because the specific surface area thereof is small and the resistance to heat is inferior.
Although the particle size of the particulate aluminum oxide is not especially restricted, the average particle size is preferably 10 xcexcm where the particulate aluminum oxide even supporting the composite oxide on the surface is hardly agglomerated and the higher dispersing ability can be maintained. When, however, the average particle size is excessive, the specific surface area is is reduced below 40 m2/g required in the co-catalyst of the present invention so that the particle size is properly determined considering the above.
The added element for forming the composite oxide of the present invention together with the cerium is at least one element selected from the group consisting of zirconium, yttrium, strontium, barium and a rare-earth element (for example, lanthanum, praseodymium, neodymium and ytterbium). The co-catalyst may be prepared by any process, and usually prepared by mixing and reacting an aqueous solution dissolving a water-soluble salt of the cerium and a water-soluble salt of the added element with a dispersion of the aluminum oxide and a reducing agent such as aluminum bicarbonate or its aqueous solution. Such a dispersion may be prepared, for example, by dissolving the water-soluble salts of the cerium and of at least one of the added elements into water and dispersing the particulate aluminum oxide thereto or by adding water to the mixture of the water-soluble salts of the cerium and of at least one of the added elements and the particulate aluminum oxide.
Although the water-soluble salt is not especially restricted, a nitrate is preferably employed for suppressing the influence caused by an impurity anion.
After the internal pores of the particulate aluminum oxide are sufficiently impregnated with the aqueous solution, the dispersion is reacted with, for example, ammonium bicarbonate or its aqueous solution functioning as a precipitating agent and a reducing agent. The mixing method includes the addition of the dispersion to the ammonium bicarbonate aqueous solution and the addition of the ammonium bicarbonate aqueous solution to the dispersion. In case of the solid ammonium bicarbonate, it may be added to and dissolved in the dispersion.
In addition to the method, another method may be employed which includes adhering the aqueous solution of the water-soluble salts of the cerium and the added element on the particulate aluminum oxide and contacting the aluminum oxide with the aqueous solution of the reducing agent for occurring the reaction.
The particulate aluminum oxide having the adhered aqueous solution can be prepared by dispersing the particulate aluminum oxide in the aqueous solution, sufficiently entering the aqueous solution into the internal pores of the particulate aluminum oxide and taking out the particulate aluminum oxide for separation by means of filtration or the like, or by placing the particulate aluminum oxide on a filtrating medium and flowing down the above aqueous solution onto the particulate aluminum oxide.
In this method, most parts of the reaction product among the cerium, the added element and the reducing agent are adhered on the particulate aluminum oxide, however, an amount of the aqueous solution which can be adhered is disadvantageously reduced. The disadvantage can be removed to some degree by increase of the concentration of the cerium and the added element in the aqueous solution and repetition of the flow-down of the aqueous solution.
The reaction product containing the cerium and the added element is deposited on the particulate aluminum oxide by any of the above-mentioned reactions. The reaction product may be a double salt, the composite oxide or the mixture thereof depending on reaction conditions. When, for example, the dispersion is added to the aqueous solution containing the reducing agent, the composite oxide is likely produced, and when the aqueous solution containing the reducing agent is inversely added to the dispersion, the double salt is likely produced.
Although the reaction temperature is not especially restricted, it has been known that the reaction product having the higher resistance to heat was obtained by the reaction under heating rather than at room temperature.
After the particulate aluminum oxide having the reaction product obtained in this manner and deposited thereon is separated by filtration or the like and washed, the particulate aluminum oxide is sintered. When the reaction product is the double salt, water and the carbonate therein are decomposed by the sintering to produce the composite oxide which is deposited on the particulate aluminum oxide. When the reaction product is the composite oxide, it is supported on the particulate aluminum oxide as it is by the sintering, thereby providing the co-catalyst.
Since the cerium is firmly supported on the particulate aluminum oxide in the form of the composite oxide different from conventional mixed power containing cerium oxide and aluminum oxide, the resistance to heat of the co-catalyst is elevated and the sintering is prevented even if the co-catalyst is exposed to a higher temperature for a longer period of time and the performance reduction due to thermal deterioration can be significantly suppressed. The coating of the particulate aluminum oxide on the composite oxide increases the contact rate with an exhaust gas.
The sintering temperature during the manufacture of the co-catalyst is not especially restricted and an ordinary sintering temperature is between 500 and 800xc2x0 C. In the present invention, the sintering is preferably conducted between 800 and 1100xc2x0 C. This is because the catalyst for purifying the exhaust gas of the internal combustion engine containing the co-catalyst actually mounted in the internal combustion engine such as an automobile is frequently exposed to a temperature exceeding 1000xc2x0 C. at maximum depending on the traveling speed of the automobile so that the higher-temperature sintering is desirable to further increasing the resistance to heat. Since, however, the co-catalyst is not the composite oxide as mentioned above in the conventional catalyst for purifying the exhaust gas of the internal combustion engine, the higher-temperature sintering of the conventional catalyst results in the deterioration of the cerium oxide which disables the smooth transfer of the oxygen, thereby inactivating the whole catalyst.
In contrast, the existence of the cerium in the form of the composite oxide together with the added element in the co-catalyst in the present invention significantly elevates the resistance to heat, and the activity reduction by the sintering at about 100xc2x0 C. is substantially negligible.
The higher-temperature sintering can hardly prevent from the reduction of the specific surface areas of the whole catalyst and the co-catalyst. The function of the added element is to secure the oxygen-absorbing and releasing abilities (oxygen storage capacity) of the cerium by forming the composite oxide. The oxygen storage capacity of the cerium generally increasing with a temperature may be decreased due to the severe sintering. Accordingly, the present invention intends not to deteriorate the complementary function for the oxidation-reduction reaction which is an essential function of the co-catalyst by maintaining the specific surface area and the oxygen storage capacity which directly affect the catalysis to be specified values or more while securing the elevation of the resistance to heat of the catalyst by the higher-temperature sintering.
As described above, the specific surface area of the co-catalyst of the present invention is adjusted to be 40 m2/gor more. The specific surface area decreases with the increase of the sintering temperature, and largely depends on the specific surface area of the particulate aluminum oxide employed rather than the sintering temperature. Accordingly, if the specific surface area of the co-catalyst after the sintering is expected to be less than 40 m2/g considering the required sintering conditions, the specific surface area of the co-catalyst is desirably adjusted to be 40 m2/g or more by employing the aluminum oxide having a smaller particle size.
Although the oxygen storage capacity of the cerium oxide is largely reduced when the cerium oxide is singly sintered at a higher-temperature, such a reduction seldom occurs when the cerium oxide forms the composite oxide as in the present invention. Although the oxygen storage capacity has a higher-temperature dependency, the functions of the co-catalyst are sufficiently displayed when the oxygen storage capacity at the practical temperature of the catalyst used as the co-catalyst for purifying the exhaust gas of the internal combustion engine is 10 xcexcmols/g or more.
In case of the co-catalyst containing the composite oxide of the cerium and the added element in accordance with the present invention, the oxygen storage capacity remarkably changes between 600 and 700xc2x0 C., and exceeds 1000 xcexcmols/g at 700xc2x0 C. Accordingly, the intrinsic oxygen storage capacity of the cerium is highly maintained by treating the exhaust gas at 700xc2x0 C. or more with the catalyst for purifying the exhaust gas of the internal combustion engine of the present invention, and further the functions required for the catalyst for purifying the exhaust gas of the internal combustion engine such as oxidation of carbon monoxide, oxidation of hydrocarbons and reduction of nitrogen oxides can be secured to promote the production of the harmless exhaust gas. Further, the functions of the co-catalyst are sufficiently displayed at 400xc2x0 C. which is in a lower-temperature range than 700xc2x0 C. when the composite oxide is formed and the above specific surface value is maintained.